Communications systems that are reliant upon Orthogonal Frequency Division Multiplexing schemes, for example Long Term Evolution (LTE) communications systems, which are sometimes referred to as 4G communications systems, are known to employ base stations, sometimes referred to as evolved Node Bs (eNode Bs) capable of communicating with User Equipment (UE) units. The UE units are typically used by subscribers to one or more cellular communications services provided by a network infrastructure that comprises a plurality of the eNode Bs to support a respective plurality of notional cells that provide wireless communications coverage for the UEs over a geographic region. The eNode Bs and the UE units are examples of communications equipment that comprise modems.
In the UE unit, a baseband IC and a Radio Frequency (RF) IC together support a transceiver architecture having a transmitter chain and a receiver chain that support operation in accordance with the different variants of the Orthogonal Frequency Division Multiplexing (OFDM) communications scheme used respectively for uplink and downlink communications. For the LTE communications system, the OFDM scheme is used in conjunction with a Frequency Division Duplexing (FDD) system, where transmission and reception takes place simultaneously albeit at different frequency bands, for example a system transmission band and a system reception band. In this type of system, the power of transmitted signals is up to 120 dB higher than the power of signals expected for reception by the receiver chain. Consequently, it is necessary to design a highly sensitive receiver chain that is capable of isolating the received signals in the presence of the transmitted signals. In the present context, the sensitivity of the receiver chain is the lowest power of a given signal that can be received successfully. In this respect, the sensitivity is determined by the architecture and design of the receiver chain and the amount of noise generated in the receiver chain impacts upon the sensitivity of the receiver chain.
When implementing a transceiver in a communications system employing an FDD scheme, the transmitter chain and the receiver chain are separated by a duplexer. This duplexer is supposed to provide sufficient signal isolation between the transmitter chain and receiver chain, which are both coupled to the duplexer and simultaneously connect the transmitter chain and the receiver chain to an antenna.
A number of technologies exist to implement a suitable duplexer, but acoustic resonator circuits, such as Surface Acoustic Wave (SAW) filters or Bulk Acoustic Wave (BAW) filters, are typically the technology of choice used to serve as the duplexer and provide the required signal isolation between the transmitter chain and the receiver chain. However, it is understood that the properties of the resonator circuits vary due to tolerances in the manufacture of the resonator circuits. Additionally, the acoustic resonator circuits are prone to suffer from frequency drift as the resonator circuits age, which can affect the cut-off frequencies of the resonator circuit over its lifetime and hence a device, such as a duplexer, employing the resonator circuit. As such, this drift impacts negatively on the signal isolation provided by the duplexer. Electronic devices and circuits are also known to be prone to performance variations caused by variations in temperature. The resonator circuit is no different in this respect, and so performance of a duplexer manufactured using the resonator circuit is subject to variations in temperature.
Manufacturers of duplexers typically publish technical specification that indicate a minimum level of performance that a given duplexer will achieve. In many cases, the duplexer is capable of exceeding the minimum level of performance published. Nevertheless, a receiver chain for operation, for example, in accordance with the LTE standard is typically designed to support the minimum level of performance of the duplexer when designing to meet performance targets set by the LTE standard of the 3rd Generation Partnership Project (3GPP).
Consequently, as in US patent publication no. 2011/0299434, it has been recognised that when the duplexer can be identified to be performing above the minimum level of performance published by the manufacturer of the duplexer, RF processing components of the UE unit can be reconfigured to use less power while still providing wireless network access in accordance with the performance targets set by the communications standard in which the UE unit is intended to operate.
In relation to US patent publication no. 2011/0299434, this document discloses a UE unit arranged to provide network access in a Universal Mobile Telecommunications System (UMTS). As mentioned above, this document recognises that duplexers typically perform better than the minimum performance specification published by the manufacturer of the duplexer. Furthermore, this document discloses the setting of the transmitter chain of the UE unit to transmit data at a maximum permitted power level and then to measure signal leakage from the duplexer to the receiver chain. The techniques described embrace factory testing and opportunistic testing, whereby the subcomponents of the UE unit responsible for testing are described as performing measurements whilst the UE unit is in service and possibly transmitting traffic-related signals in the communications network. However, in such communications networks, transmit power is controlled by the Node B with which the UE unit is communicating and transmission by the UE unit of signals at power levels greater than the power level instructed by the Node B contravenes the UMTS standard and if applied in the context of the LTE standard would also contravene the LTE standard, because use of such elevated power levels can render the Node B/eNode B incapable of receiving signals from other Node B's/eNode B's in the communications network. The power consumption of the transceiver of the UE unit is described as being controlled by setting different sub-circuits to low-power modes based on signal leakage measurements made. The settings are predetermined and recorded in a look-up table having different settings for different parts of the transceiver.